In the past, a variety of magnetic materials and structures have been utilized to form magnetoresistive materials for non-volatile memory elements, read/write heads for disk drives, and other magnetic type applications. One prior magnetoresistive element utilized a magnetoresistive material that has two magnetic layers separated by a conductor layer. The magnetization vectors of the two magnetic layers typically are anti-parallel to each other in the absence of any magnetic fields. The magnetization vectors of one of the layers points in one direction and the magnetization vector of the other layer always points in the opposite direction. The magnetic characteristics of such magnetic materials typically require a width greater than one micron in order to maintain the orientation of the magnetization vectors along the width of the cell. The large width requirement limits the density of memories utilizing such materials. Additionally, reading the state of such memories typically requires a two-phase read operation that results in very long read cycles. The two phase read operation also requires extra circuitry to determine the state of the memory, thus increasing the cost of such memories. An example of such a magnetic material and memory is disclosed in U.S. Pat. No. 4,780,848 issued to Daughton et al. on Oct. 25, 1988.
Another prior material uses multi-layer giant magnetoresistive materials (GMR) and utilizes submicron width, in order to increase density. In this structure the magnetization vectors are parallel to the length of the magnetic material instead of the width. In one embodiment the magnetization vector of one magnetic material layer is always maintained in one direction while the magnetization vector of the second magnetic layer switches between parallel and antiparallel to the first vector in order to represent both logical "0" and "1" states. In order to determine the logical state of a memory cell utilizing this material, the memory cell has a reference cell and an active cell. The reference cell always provides a voltage corresponding to one state (either always a "1" or always a "0"). The output of the reference cell is compared to the output of the active cell in order to determine the state of the memory cell. The requirement for an active and a reference cell reduces the density of a memory that utilizes such elements. Additionally, each memory cell requires transistors to switch the active and reference cells at appropriate times in order to read the cells. This further increases the cost of the memory.
A magnetic random access memory (MRAM) is a nonvolatile memory which basically includes a giant magnetoresistive (GMR) material, a sense line, and a word line. The MRAM employs the GMR effect to store memory states. Magnetic vectors in one or all of the magnetic layers of GMR material are switched very quickly from one direction to an opposite direction when a magnetic field is applied to the GMR material over a certain threshold. According to the direction of the magnetic vectors in the GMR material, states are stored, for example, one direction can be defined as a logic "0", and another direction can be defined as a logic "1". The GMR material maintains these states even without a magnetic field being applied. The states stored in the GMR material can be read by passing a current through the cell in a sense line, because of a difference between the resistances of the two magnetic states. Since GMR materials ire composed of very conductive non-magnetic layers (generally copper or the like) and conductive magnetic layers (e.g., Fe, Ni, Co, etc.), the sheet resistance of the material is relatively low (e.g. 15 ohms/square) and the change in resistance is relatively small (e.g. 5%). Consequently, the resistance difference between the two states for a cell with length-to-width ratio of ten is only about 7.5 ohms.
Accordingly, it is highly desirable to provide magnetic random-access memories and memory cells which have higher resistance differences between the two magnetic states and thus are easier to utilize.
It is a purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell.
It is another purpose of the present invention to provide a new and improved array of multi-state, multi-layer magnetic memory cells.
It is still another purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell which is simpler to manufacture and to use.
It is a further purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell which utilizes less sensing current and produces a larger change between states.
It is still a further purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell which is faster and simpler to sense.